CHITOSAN-BASED ADSORBENTS FOR THE REMOVAL OF METAL … · (Bahan Penjerap Berasaskan Kitosan untuk Penyingkiran Ion Logam dari Larutan Akueus) Zetty Azalea Sutirman1, Mohd Marsin

Malaysian Journal of Analytical Sciences, Vol 22 No 5 (2018): 839 - 850

DOI: https://doi.org/10.17576/mjas-2018-2205-11

839

MALAYSIAN JOURNAL OF ANALYTICAL SCIENCES

Published by The Malaysian Analytical Sciences Society

CHITOSAN-BASED ADSORBENTS FOR THE REMOVAL OF METAL IONS

FROM AQUEOUS SOLUTIONS

(Bahan Penjerap Berasaskan Kitosan untuk Penyingkiran Ion Logam dari Larutan Akueus)

Zetty Azalea Sutirman1, Mohd Marsin Sanagi

1,2*, Khairil Juhanni Abd Karim

1, Ahmedy Abu Naim

1,

Wan Aini Wan Ibrahim1,2

1Department of Chemistry, Faculty of Science 2Centre for Sustainable Nanomaterials, Ibnu Sina Institute for Scientific and Industrial Research

Universiti Teknologi Malaysia, 81310 UTM Johor Bahru, Johor, Malaysia

*Corresponding author: [email protected]

Received: 16 April 2017; Accepted: 7 March 2018

Abstract

Wastewater containing heavy metal ions is one of the most serious environmental concerns. Exposure to elevated levels of heavy

metals can adversely affect water resources, endangering the ecosystems and human health. Among the various treatment

technologies, adsorption using biopolymer seems a promising alternative method. Chitosan is a natural polymer produced from

chitin with excellent properties such as biocompatibility, biodegradability and non-toxicity. Moreover, chitosan is known as an

effective sorbent due to the presence of amino and hydroxyl groups in its molecules which can serve as attachment sites towards

metal ions. Recently, chitosan derivatives as metal ion sorbents have gained considerable attention. These derivatives are

prepared by either physical or chemical modifications or both in order to improve chitosan properties in adsorption. This paper

discusses recent developments in the modifications of chitosan and the application of the derived materials in the removal of

metal ions from aqueous solutions. The mechanisms of adsorption, metal sorption capacities, effect of pH, isotherm and kinetic

models are also described.

Keywords: chitosan, modification, sorption, metal ions

Abstrak

Air sisa yang mengandungi logam berat merupakan salah satu isu alam sekitar yang serius. Pendedahan yang tidak terkawal

kepada logam berat boleh menyebabkan kesan negatif terhadap sumber air, membahayakan ekosistem dan kesihatan manusia. Di

antara pelbagai teknologi rawatan, penjerapan menggunakan biopolimer menunjukkan kaedah alternatif yang menjaminkan.

Kitosan ialah polimer semula jadi yang dihasilkan daripada kitin dengan ciri-ciri seperti biokompatibiliti, biopenguraian dan

bukan toksik. Tambahan pula, kitosan dikenali sebagai penjerap yang efektif kerana mempunyai kumpulan amina dan hidrosil

dalam molekulnya yang bertindak sebagai tapak penghubung terhadap ion logam. Baru-baru ini, derivatif kitosan sebagai

penjerap ion logam telah menerima perhatian yang luas. Derivatif-derivatif ini dihasilkan sama ada melalui pengubahsuaian

fizikal atau kimia atau kedua-duanya untuk menambah baik ciri-ciri kitosan dalam penjerapan. Kertas ini membincangkan

pengembangan yang terbaru dalam pengubahsuaian kitosan serta aplikasi daripada bahan derivatif dalam penyingkiran ion logam

dari larutan akues. Mekanisma penjerapan, kapasiti penjerapan logam, kesan pH, model dan kinetik juga diterangkan.

Kata kunci: kitosan, pengubahsuaian, penjerapan, ion logam

Introduction

Heavy metals such as cadmium (Cd), mercury (Hg), lead (Pb), zinc (Zn), arsenic (As), nickel (Ni) and chromium

ISSN

1394 - 2506

Zetty Azalea et al: CHITOSAN-BASED ADSORBENTS FOR THE REMOVAL OF METAL IONS FROM

AQUEOUS SOLUTIONS

840

(Cr) are elements with atomic density of at least five times the specific gravity of water [1]. They are also known as

trace elements due to their presence at low concentrations (<1000 ppm) in environmental matrices. In fact, some of

them have been recognized as essential nutrients for organisms (i.e. Cu and Zn).

Even though heavy metals are natural components of the Earth’s crust, most of environmental pollution and human

exposure to heavy metals are caused by various anthropogenic activities such as military, industrial and agricultural

processes and waste disposal. Heavy metals are also released to the ecosystem through natural processes like

weathering of rocks and volcanic eruptions [2, 3]. Most heavy metals are highly toxic to living organisms especially

when their concentrations exceed the allowable limits in the ecosystem. These pollutants are non-biodegradable and

can be absorbed by marine life. Once high concentrations of heavy metals enter the food chain, they tend to

accumulate in living tissues and thus, dysfunction the system of the human bodies [4]. Hence, it is important to

decontaminate or minimize these heavy metals from water sources in the interest of public health and the

environment.

Up to now, various treatment methods have been developed to remove heavy metals, which include chemical

precipitation, solvent extraction, ion exchange, evaporation, reverse osmosis, electrolysis and adsorption. However,

most of these methods tend to show some economical and technical disadvantages [5]. For example, chemical

precipitation has been widely used to remove dyes or heavy metals from inorganic effluent by increasing the pH of

the solution to convert the soluble substances into an insoluble form. Even though the process is simple, it generates

a large quantity of sludge which requires further treatment and high cost. Chemical precipitation is also not effective

to remove trace level of pollutants from aqueous solutions. Meanwhile, in ion exchange method, the resin, either

synthetic or natural solid, exchanges its cations with the unwanted substance in the wastewater. The problem is

drawn when the ion exchanger is quickly polluted and thus, reducing the exchange capacity. In addition, this method

usually consumes high capital and operational costs. Coagulation-flocculation has been capable of removing

pollutants from solution, but the process can destabilize colloidal particles by adding a chemical agent (coagulant)

which results in sedimentation [6, 7]. Adsorption is established as the most effective method for water

decontamination applications and analytical separation purposes. It offers advantages over the other options such as

simplicity in terms of design and operation, uses less chemicals, requires low initial costs and can remove different

types of pollutants [4, 8].

Chitosan, an amino polysaccharide produced from chitin are found naturally in some fungi. It is the most versatile

biopolymer for a broad range of applications due to its biocompatibility, biodegradability and antibacterial property.

Furthermore, this material is regarded as an ideal adsorbent. The presence of –NH2 and –OH groups on the

polymeric backbone can serve as chelating and reaction sites [9].

Several methods have been employed to modify chitosan either physically or chemically to enhance adsorption

capacity. Crosslinking is one of the common modifications performed on chitosan mainly to prevent its dissolution

in acidic media [10]. This technique involves the formation of covalent bonds between two polymer chains by using

bifunctional reagents containing reactive end groups that react with functional groups of chitosan (i.e. NH2).

Therefore, it must be taken into account that crosslinking can reduce the adsorption capacity as it diminishes the

number of free amino on chitosan, but this loss is worth to increase the stability of the polymer [11].

Another important modification is graft copolymerization. This approach has gained considerable interest among

researchers and has become a vital technique in polymer chemistry. Grafting reaction provides various molecular

designs leading to novel types of hybrid materials, which are composed of bio- and synthetic-polymers [12].

Chitosan has two types of reactive groups; hydroxyl and amine groups for grafting to occur. Different functional

groups have been successfully grafted onto chitosan by covalent bond onto the chitosan backbone.

Several reviews on chitosan modifications have been reported particularly on metal ions removal from water of

wastewater [10, 13-16]. Other works on chitosan and its derivatives not limited to metal sorption have also been

reviewed [17-21]. However, to the best of our knowledge, no detail review has been reported on different

modifications of chitosan such as beading, crosslinking and grafting and their adsorption potential for various metal

ions. In this paper, we present updated relevant information on the current modifications techniques employed on



841

chitosan to enhance its properties in metal ions adsorption. The adsorption performances of chitosan-based

adsorbents are also compared.

Chitosan

Chitosan, a main derivative of chitin, can be prepared by deacetylation of chitin under alkaline conditions (Figure

1). It was first discovered by Professor C. Rouget in 1859. Chitosan only occurs naturally in some fungi such as

Mucor rouxii and Choanephora cucurbitarum. This polysaccharide is mainly composed of a linear of β (1→4)-

linked 2-amino-2-deoxy-β-D-glucopyranose. Since complete deacetylation of chitin is practically impossible, the

molar ratio of N-acetyl-2-amino-2-deoxy-ᴅ-glucose (GlcNAc) is often expressed as a degree of N-acetylation (DA),

which defines the average number of GlcNAc units per 100 monomers. Meanwhile, the deacetylation degree (DD)

is used for 2-amino-2-deoxy-D-glucose (GlcN) residue [22].

OO O

O

NHHO

HO NH

OH

OH

OO

HO NH2

OH

OO O

O

NH2HO

HO NH

OH

OH

OO

HO NH2

OH

C O

CH3

C O

CH3

C O

CH3

N-deacetylation

chitin

chitosan

GlcNGlcNAc

Figure 1. Chemical structures of chitin and chitosan

Chitosan is named by the content of DD in the polymer. The DD for commercial chitosan is usually 70-95%. Some

researchers believe that chitosan is built up of at least 50% DD and soluble in aqueous acidic media [23, 24]. The

solubility of chitosan occurs when the amino group, –NH2 on the GlcN repeated unit is protonated, converting the

polysaccharide to polyelectrolyte (pKa=6.5). However, an exact nomenclature with respect to DD ratio between

chitin and chitosan has not been clearly defined because it may vary depending on the production process, natural

source and other factors [22].

The effectiveness of chitosan and its derivatives to interact with metal ions from aqueous solution has been studied

by many workers. It is important to note that the adsorption capacities of these adsorbents depend on the experiment

conditions such as pH, initial concentration, contact time, adsorbent dosage, presence of competitive ions, etc.

Modified chitosan

Although chitosan shows a great potential as sorbent, it has severe drawbacks that restrict its application in water

treatment and purification. Chitosan is commonly available in powder or flake form as the end product in the

production process. It has crystallized structure due to strong inter- and intra-molecular hydrogen bonding in the


AQUEOUS SOLUTIONS

842

structure, presents poor acid resistance and low porosity. In view of these limitations, raw chitosan is not suitable to

be used in adsorption process without further modifications [25].

Modification of chitosan in adsorption study has two main goals. The first goal is to increase the chemical stability

of the polymer in wide pH-range aqueous media. The second goal is to improve the sorption performance such as

sorption capacity and pollutant selectivity [13]. The modification of chitosan can be carried out by either physical or

chemical techniques or both.

Physical modification

The selection of physical modification of chitosan depends mainly on the system configuration to be used for

applications. In this manner, several techniques have been employed to mold chitosan into different morphologies

such as powder, microspheres, hydrogels, membranes and fibers [26].

Solid chitosan can be obtained when chitosan in acidic aqueous solution is mixed with alkali. This method is usually

used to prepare membranes, fibers and foremost, spherical gel beads of different sizes and porosities. Adarsh and

Madhu [27] have prepared the beads by spraying chitosan solution into a precipitation bath containing 1 M NaOH.

In order to obtain membrane, chitosan solution was spread on a petri dish and dried. The membranes were then

immersed in NaOH solution for neutralization [28]. Chitosan membrane or film can also be fabricated by the

solvent evaporation method.

Furthermore, porous three-dimensional sponges can be prepared by freeze–drying. In this method, chitosan

solutions or gels are frozen followed by lyophilization. The porosity and morphology of the material produced

depends on the chitosan molecular weight and on the composition and concentration of the starting solution, and

most importantly on the freezing temperature and freezing rate [29].

Chemical modification

Chitosan bears a large number of free hydroxyl and amino groups distributed along the backbone, which are an

ideal candidate for chemical modification. The main aim of modifying chitosan is to vary properties such as

solubility, water sorbency, adsorptive capability and thermal resistance. Chemical modification, however, would not

change the fundamental skeleton of chitosan, yet affords a wide range of derivatives with desired properties for

specific use and applications in various areas mainly of pharmaceutical, biomedical and biotechnological fields [10].

Derivatization by incorporating functional groups to chitosan may involve the –NH2 group at the C-2 position

(specific reactions) or –OH groups at the C-3 and C-6 positions (non-specific reactions) [30].

Preparation of new chitosan-based materials may include impregnation, crosslinking, graft polymerization and

composite formation [18]. The use of chitosan in relatively low pH solution is not feasible because it tends to form

gel or dissolve. Thus, one way to overcome this problem is by inducing crosslinking on chitosan. Crosslinking

agents or crosslinkers are generally bifunctional compounds with various forms such as straight chains, branch

chains and rings. These crosslinking agents are classified upon their nature and interaction with biopolymer (i.e.

chemical or physical crosslinking). Chemical crosslinking occurs when the crosslinker forms intermolecular

covalent bond between the polymer chains. The reaction often involves the -NH2 and -OH functional groups on the

biopolymer. Chemical crosslinking is commonly accomplished using crosslinker such as glutaraldehyde,

epichlorohydrin, ethylene glycol diglycidyl ether, genepin, glyoxal, cylclodextrins, etc. [10, 31]. Meanwhile,

physical crosslinking results from non-covalent linkages between polymer chains. The reaction is weaker than

chemical crosslinking since it relates to physical attractive forces such as ionic interaction, hydrophobic and

hydrogen bonding. Pentasodium tripolyphosphate and calcium chloride are the two established examples of

physical crosslinker. In a general way, crosslinking decreases the adsorption capacity slightly because amine groups

of chitosan, which is known to be the main sites for target analytes, are usually bound with the crosslink agent to

form a stable covalent bond (i.e. imine group).

Grafting copolymerization is another common way to improve adsorption capacity and increase chelating or

complexation properties by introducing functional groups onto chitosan. During grafting reaction, side chains are

covalently bonded to the main polymer backbone to give a branched structure copolymer. Until today, many



843

research works have been carried out to synthesize graft copolymers by polymerization reaction of chitosan with

monomer using free radical initiation. There are two main kinds of free radical initiating systems employed. The

first type of initiation is called conventional or chemical initiation. In this system, the initiators used are usually

thermal initiator and redox initiator. Meanwhile, the second type of initiation is free radical initiation by irradiation

with either high energy (i.e. γ-ray) or low energy (i.e. microwave, UV light) [32].

Chitosan beads and membranes

Many studies have reported that the efficiency of adsorption strongly depends on the physicochemical properties of

adsorbents including crystallinity, surface area, porosity and particle size. Therefore, chitosan flake or powder is not

considered to be used as adsorbent due to its high crystallinity, resistance to mass transfer, low adsorption capacity

and very low porosity. Small particle is also inappropriate for a fixed bed column system especially in industrial

scale because they can increase their resistance to mass transfer rate, lead to clogging effect as well as

hydrodynamic limitations [17]. Conversion of chitosan flake into bead has been reported to be essential as it allows

the expansion of the polymer network, thus increasing access to internal sorption sites and enhancing diffusion

mechanism.

Chitosan, in its native state, is characterized as a crystallized polymer. Since adsorption process usually occurs only

at the amorphous region of the crystal, chitosan is usually subjected to physical modification by converting it into

gel beads in order to reduce its crystallinity, thus enhancing the adsorption capacity. Cahyaningrum et al. reported

the comparison between chitosan powder and chitosan beads for the removal of Hg(II) ions [33]. The results

revealed that the adsorption capacity of the metallic ion by chitosan beads (17.39 × 10-4

mol g-1

) was much higher

than that of chitosan powder (7.20 × 10-4

mol g-1

).

Wan Ngah et al. compared the adsorption capacity of chitosan flakes and beads as sorbents for removing Pb(II) ions

from aqueous solution [34]. They found that the maximum adsorption capacity of chitosan beads was higher than

that of chitosan flakes because the structure of the beads slightly changed because of reducing the crystallinity of

chitosan. The adsorption of Cu(II) ions onto flake- and bead-type chitosan derived from shrimp was compared by

Wu et al. [35]. The results showed that chitosan flakes possessed higher uptake for Cu(II) ions than that of chitosan

flakes.

Crosslinked chitosan

As afore-mentioned, raw chitosan has a major limitation of being highly soluble in most dilute mineral and organic

acid solutions by protonation of the amine groups. This makes it even more difficult to evaluate its application as

sorbent in the treatment of industrial effluents. Crosslinking process has been found to be an effective method to

reinforce chemical stability of chitosan under acidic conditions. Several common chemical crosslinking agents such

as glutaraldehyde (GA), epichlorohydrin (EPC), triphosphate (TPP) and ethylene glycol diglycidyl ether (EDGE)

have been used for this purpose. Results on metal ions adsorption onto crosslinked chitosan using different

crosslinking agent are shown in Table 1. It is important to note that the adsorption capacities of the adsorbents cited

in this paper depend on the experiment conditions such as pH, initial concentration, contact time, adsorbent dosage,

presence of competitive ions, etc.

Wan Ngah and Fatinathan successfully crosslinked chitosan beads with GA for removal of Cu(II) ions in aqueous

solution [9]. The crosslinked chitosan was insoluble in acetic acid solution, showing crosslinking reaction enhanced

the resistance of the polymer against acid. The researchers also investigated the effect of GA content in adsorption.

An increase in the density of crosslinking lowered the adsorption capacity of chitosan because the number of amino

groups that serve as binding sites for metal ions was reduced.

In another work, a batch experiment was carried out to study the adsorption of Fe(II) and Fe(III) ions by chitosan

and crosslinked chitosan beads [43]. The chitosan beads were crosslinked with GA, EPC and EDGE in order to

improve its chemical resistance and mechanical strength. The maximum adsorption capacity was in the sequence of

chitosan > chitosan-EPC > chitosan-GA > chitosan-EDGE.


AQUEOUS SOLUTIONS

844

Table 1. Removal of different metal ions from aqueous solutions by crosslinked chitosan

Crosslinking Agent Metal

Ion

qmax

(mg g-1

)

pH Isotherm Reference

TPP Cu(II) 200.0 4-5 L [36]

GA

Pb(II)

Cd(II)

24.2

23.9

5-7

L

L

[37]

GA crown ether

Cd(II)

Hg(II)

Pb(II)

-

126.4

96.8

2

6

6

-

-

-

[38]

[39]

GA 2-aminopyridine glyoxal Schiff’s base

Pd(II)

Cu(II)

Cd(II)

Ni(II)

166.7

124

84

67

8

5

5

5

L

L

L

L

[40]

[41]

Adipic acid dihydrazide Cu(II) 200.1 5 L [42]

L = Langmuir

Chitosan film was crosslinked using two different crosslinkers; GA and EPC for the adsorption of Cu(II), Hg(II) and

Cr(II) [44]. In this work, X-ray photoelectron spectroscopy (XPS) confirmed the crosslinking of chitosan with GA

and EPC occurred preferentially on hydroxyl and amino groups thus forming final structures with different

functional groups. The uptake of Cu(II) and Cd(II) ions by chitosan crosslinked EPC subsequently treated with TPP

was studied by Laus and Fevere [45]. The results for binary solutions showed the adsorbent has great affinity

towards Cu(II) rather than Cd(II) due to a significant competition effect.

Grafting of chitosan

Grafting of chitosan allows the formation of functional derivatives by covalent binding of molecules onto chitosan

backbone. This kind of modification can lead to increasing functional sides which involve in metal complexation.

The adsorption capacities of grafting chitosan for the removal of different metal ions from aqueous solutions are

listed in Table 2.

Table 2. Removal of different metal ions from aqueous solutions by grafted chitosan

Graft Copolymer Metal

Ion

qmax

(mg g-1

) pH Isotherm Reference

Polyaniline grafted chitosan

Silane grafted chitosan

Succinyl-grafted chitosan

Poly(methacrylic acid)-grafted chitosan

Ethylacrylate grafted chitosan

N-butylacrylate grafted chitosan

Amidoximated chitosan-grafted polyacrylonitrile

Pb(II)

Cd(II)

As(V)

Zn(II)

Cd(II)

Pb(II)

Cd(II)

Zn(II)

Cr(II)

U(IV)

16.1

14.3

11.8

1401

146.1

613

573

653

17.2

312.1

6

6

5

5

6

6

6

3.5

~7

L

L

L

T

L

L

L

L

L/F

L

[46]

[47]

[48]

[49]

[50]

[51]

[52]

L = Langmuir, T = Tóth and F = Freundlich



845

Graft copolymer of chitosan with acrylamide was prepared using gamma radiation and used for the adsorption of

copper(II) and nickel(II) from aqueous solutions [53]. The adsorption of both metal ions increases with increasing

pH and polymer concentration. At lower pH, hydrogen ions occupy most of the adsorption sites of the adsorbent and

this leads to a very low adsorption due to electrostatic repulsion. Lalita et al. was grafted chitosan with N-

isopropylacrylamide with the presence of azoisobutrylnitrile (AIBN) as an initiator [54]. Chitosan and the graft

copolymer were characterized by different techniques such as by FTIR, TGA/DTA, XRD and SEM. The graft

copolymer showed good adsorption capacity towards Cr(VI), Cu(II) and Fe(II) ions.

Some researchers reported the enhancement of adsorption capacity after grafting due to introducing of new

functional group onto chitosan chain. Galhoum et al. found that grafting of aspartic acid onto chitosan significantly

increased the sorption properties for Nd(III) [55]. The adsorption capacity was two times higher compared to that of

chitosan and GA-crosslinked chitosan. Similarly, the adsorption capacities of chitosan for several metal ions (Pb(II),

Hg(II), Cd(II), Ni(II), Mn(II), Co(II), As(V) and Se(VI)) were increased after grafting with acrylic acid [56].

Grafting of crosslinked chitosan

Grafting copolymerization on crosslinked chitosan beads with vinyl monomers is necessary to increase the number

of adsorption sites and thereby, the adsorption capacity. For example, adsorption of Pb(II) and Cd(II) was carried

out by Benamer et al. on chitosan beads grafted with acrylic acid [57]. Chitosan was first crosslinked with

glutaraldehyde and then grafted with monomers using 60

Co gamma radiation technique. The graft copolymer

presented greater ability to adsorb Cd and Pb ions compared to unmodified chitosan. The adsorption capacity of the

graft copolymer also increased with the increasing degree of grafting.

Our research group utilized ammonium persulfate as free radical initiator for grafting methacrylamide onto

crosslinked chitosan beads. Several characterization techniques such as FTIR, TGA, SEM and 13

C NMR were used

to confirm the successful modifications. The efficiency of the beads as adsorbent was studied to remove Pb(II) ions

in batch experiment [58]. On the other hand, we successfully grafted N-vinyl-2-pyrrolidone (NVP) onto crosslinked

chitosan by using the same technique (unrevealed data). The beads have shown a good adsorption capacity towards

several metal ions such as Pb(II), Cu(II) and Cd(II). The interaction between adsorbent and Cu(II) ions is also

proposed according to XPS analysis (Figure 2).

Figure 2. Possible interactions between Cu(II) ion and crosslinked chitosan-g-poly(NVP)

The effect of experimental conditions on the adsorption and desorption behavior of Cd(II) and Pb(II) on polyaniline

grafted cross-linked chitosan beads was studied by Igberasa and Osifo [59]. The study showed that the binding of

metal ions was highly dependent on factors such as pH, adsorbent dosage, contact time, initial concentration and

point of zero charge.

Novel chitosan-based adsorbent was synthesized for Fe(II) adsorption by Liu et al. from TPP and GA-crosslinked

chitosan, followed by grafting with five phenolic acid derivatives (Gallic acid (GA), protocatechuic acid (PA),


AQUEOUS SOLUTIONS

846

caffeic acid (CA), p-hydroxybenzoic acid (HA), vanillic acid (VA)) [60]. Results showed that the graft

copolymerization occurred at amino groups (C2), hydroxyl groups (C3 and C6) of chitosan and incorporation of

carboxyl groups from phenolic acids decreased in crystallinity of the polymer. In addition to the above-stated

examples, several other grafting crosslinked chitosan sorbents are presented in Table 3.

Table 3. Removal of different metal ions from aqueous solutions by grafting crosslinked chitosan

Chitosan Derivative CA Metal

ion

qmax

(mg g-1

) pH Isotherm Ref.

Crosslinked chitosan-g-acrylonitrile

Itaconic acid grafted crosslinked chitosan beads

Crotonic acid grafted crosslinked chitosan beads

Chitosan beads grafted with acrylic acid

Poly(acrylic acid) grafted and crosslinked chitosan

Poly (itaconic acid)-grafted crosslinked chitosan

nanoadsorbent

Poly(maleic acid)-grafted crosslinked chitosan

Triethylene-tetramine grafted magnetic chitosan

GA

EPC

GA

GA

GA

GA

EPC

Cu(II)

Cu(II)

Pb(II)

Cu(II)

Pb(II)

Hg(II)

Pb(II)

Hg(II)

Pb(II)

230.8

19.6

15.9

217.4

224.7

734.3

870.1

1320

1044

370.3

5

6

7

4

5

6

5

6

6

L

F

F

L

L

L

L

L

L

L

[61]

[62]

[63]

[64]

[65]

[66]

[67]

CA= crosslinking agent, L = Langmuir and F = Freundlich

Conclusion and Future Perspective

Recent years have seen increasing developments and works carried out to find alternatives in view of economic

adsorbents for water treatment. Natural polymers, in particular chitosan, have high potential as sorbent in removing

a wide range of contaminants, yet this material presents significant limitations which require appropriate

developments. Raw chitosan (powder or flakes form) has crystalline structure, consequently limit its adsorption

capacity as adsorption merely takes place on the amorphous of the crystal region. This problem is solved by

converting chitosan into hydrogel beads through gel formation process. However, chitosan beads tend to dissolve in

acidic medium. The stability of the beads can be enhanced by crosslinking reaction. Crosslinking agent is usually

prone to react with the amine group in chitosan which known to be main chelating sites for metal ions. Therefore,

grafting of functional groups onto chitosan backbone is necessary to increase selectivity and capacities. Grafting

appears to be a convenient approach for improving the intrinsic properties of natural polymers or bring them with

new properties.

Modifications of chitosan by crosslinking and grafting represent good alternatives for new potential adsorption

system. Yet, there are still few factors that need to be considered to transfer the process to industrial scale. The

influence of crosslinker content on the reaction or the properties of chitosan (i.e. crystallinity, solubility) could be

investigated in future study to prepare a better adsorbent. Developing adsorbents that can remove variety of

pollutants from industrial effluents is preferable. It would also be of interest to investigate other type of modified

adsorbents and their capable in treating wastewater.

Acknowledgement The authors would like to thank Universiti Teknologi Malaysia for facilitations and the Ministry of Higher

Education Malaysia (MOHE) and Organization for the Prohibition of Chemical Weapon for financial supports

through vote numbers Q.J130000.2509.09H84 and R.J130000.7309.4B222, respectively.



847

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